One of the more interesting announcements to come out Blizzcon t his year is that Destiny 2, the base game, is currently free from now until November 18th. Claim the game before then, and it’s yours to keep forever. After that, if you like what you’ve played, you’ll still need to pick up the Forsaken expansion, but all other previously released DLC expansions are included with that.
There’s also going to be a free Gambit weekend running from the 9th to the 11th. Gambit is a Forsaken exclusive PvPvE mode that would otherwise require the expansion to play. The game should be free to claim for anyone with a Battle.net account.